Liquid electrostatic printing suffers from the inherent nature of the toner changing its properties during the course of usage. For example, the conductivity and/or charging of the toner changes while being used to make prints. Techniques in common usage today correct writing head parameters, such as laser power, developer voltage (the charging on the developer), photoreceptor charging and possibly the look up tables and screen sets to compensate for changes in the toner and thusly in order to keep the final output (e.g. the prints) constant. In general a desired value of charging of the toner is set a priori and the other parameters are varied to provide an optimum or at least an acceptable image. Then the charging of the toner is controlled to preserve image quality. Control of the charge director component of the toner in response to a conductivity measurement is sometimes used to modify toner charging, such as described in U.S. Pat. No. 4,860,924 to Simms, et al., the disclosure of which is incorporated herein by reference. A low field conductivity measurement, which is a measure of a current between electrodes immersed in the toner reservoir, is often used.
During operation the conductivity of the toner is monitored. The system is purposely unbalanced so that the charge level falls slowly with use. Charge director is added to increase the charge on the toner particles, when the measured conductivity reaches a lower threshold level. From time to time, a calibration step is carried out, to adjust the developer voltage and laser power to optimize image quality for the particular batch and condition of the toner. Using a system in which the target value of toner low field conductivity is set, the developer voltage and laser power is allowed to vary during the periodic calibrations. It is noted that between calibrations the voltage and laser power remain constant and only the conductivity value is controlled.
These corrective measures come with their own problems such as increased background development due to high developer voltage, inhomogeneous solid print due to low developer voltage, varied developed spot shape due to highly variable developer voltage, expensive lasers must be used due to the demands of highly variable power requirements, and/or hard-to-gauge correlation between measured area cover versus digital input cover which leads to unstable color and line-work, just to name a few. In general these variations will only take place from time to time.
An additional problem is that, while this system does give good image quality, there are slight variations in the color balance of images between different printers and between different batches of toner in the same printer, as well as slight variations with time.